A direct digital-RF transmitter (TX) has several advantages compared to digital-analog-RF transmitters. The direct digital-RF transmitter arranges the digital-analog interface close to the antenna so that fewer analog components are required. The typical analog issues like in-phase and quadrature-phase (IQ) mismatch, local oscillator (LO) leakage; image distortion can be largely alleviated and even avoided. The direct digital-RF transmitter also enhances the system flexibility through multi-mode and multi-band operation enabled by agile digital signal processing. In addition, the direct digital-RF transmitter can take advantage of the increasing speed and density of digital processing, and high level integration. Thus, the direct digital-RF transmitters have benefits for both base-station and mobile applications.
The direct digital-RF transmitter includes a switching mode power amplifier (SMPA), such as a class-D or class-S power amplifier, employing a particular power coding scheme, such as DSM (delta sigma modulation), PWM (pulse width modulation) and PPM (pulse position modulation), in addition with a reconstruction band-pass filter (BPF).
In terms of power, the RF power amplifier (PA) consumes the most energy in the transmitter. A main advantage of this transmitter is that, the SMPA is always between ON (saturated) and OFF (cut-off) operating regions, achieving high peak efficiency. However, if non-constant envelope signals, which are common for 3rd generation (3G) and 4th generation (4G) cellular mobile communication systems, are encoded into the single bit digitized signals, then the in-band power over the entire digitized signal power, defined as the power coding efficiency, is low, because the generation of quantization noise is inevitable and widely spread throughout the frequency domain due to a noise shaping function, which is required from the system linearity specification. Because this noise signal is also amplified by the SMPA, the unwanted noise power becomes wasteful, which causes both excessive power loss and total TX efficiency degeneration.
This problem is present in the band-pass delta-sigma modulation (BPDSM) based class-S power amplifiers. See, e.g., U.S. 2003/0210746, U.S. 2006/0188027, EP 2063536, and U.S. Pat. No. 7,825,724.
The total TX power efficiency is related to the power coding efficiency of the encoder as well as the power efficiency of the PA. The PA efficiency ηPA, which depends on the PA circuit design, is usually relatively high (>80%) for SMPA at saturated power level. In contrast, the power coding efficiency ηCODE is the direct measurement of the power spectral density (PSD) of the encoder pulse train p(t) and is based on the performance of the encoder. The performance of the encoder is relatively low (<20%) for conventional power coding schemes with non-constant envelope modulated signals. Therefore, the power coding efficiency ηCODE is the primary concern for direct digital-RF transmitter, which sets the upper bound of the entire efficiency of the transmitter. To increase the efficiency of direct digital-RF transmitters under modulated non-periodic switching conditions, the power coding efficiency needs to be improved.
The low power coding efficiency is a result of noise shaping in a delta sigma power coding scheme. Thus, some conventional coding schemes use various PWM techniques to improve the power coding efficiency.
For example, Blocher et al., “Coding efficiency for different switched-mode RF transmitter architectures,” Circuits and Systems, 2009. MWSCAS '09. 52nd IEEE International Midwest Symposium on, vol., no., pp. 276, 279, 2-5 Aug. 2009, describe a polar PWM architecture. The envelope of the baseband signal is modulated in a PWM encoder, where the PWM encoding is performed by comparing the envelope magnitude with a reference waveform (triangular or saw-tooth). Typically, the frequency of the PWM reference waveform is 10-100 times the baseband bandwidth of the input signal. This architecture can achieve high power coding efficiency and required linearity, but is hard to implement digitally. In addition, since the transmitted signal is combined directly at the RF carrier frequency, the time alignment between amplitude and phase is difficult, especially for wideband signals.
Another power coding efficiency enhancement approach is RFPWM, described in Raab, F. H., “Class-D power amplifier with RF pulse-width modulation,” Microwave Symposium Digest (MTT), 2010 IEEE MTT-S International, vol., no., pp. 924, 927, 23-28 May 2010. The output signal of RFPWM includes 2-level (unipolar or bipolar-NRZ) or 3-level waveform (bipolar-RZ) per RF carrier period. First, both baseband in-phase (I) and quadrature (Q) are up-converted into RF domain. The magnitudes of RF Cartesian signals are encoded by the varied pulse width to generate pulse width modulated RF signal. By this method, any complex input signal can be mapped to a time-continuous and amplitude-discrete output signal suited for switch-mode amplification. However, this encoding is also processed by analog/RF high speed comparators, which is usually cost and energy hungry. Therefore, the RFPWM encoder is suitable to the low carrier frequency like the class-D power amplifiers for audio applications, but not fit for RF transmitter applications at GHz.
Other PWM power coding schemes for digital implementation include pulse-position modulation (PPM), e.g., PWM/PPM scheme described in U.S. Pat. No. 6,993,087 and pulse width position modulation (PWPM) described in Thiel, B. T.; Dietrich, S.; Zimmermann, N.; Negra, R., “System architecture of an all-digital GHz transmitter using pulse-width/position-modulation for switching-mode PAs,” Microwave Conference, 2009. APMC 2009. Asia Pacific, vol., no., pp. 2340, 2343, 7-10 Dec. 2009. Similar to the polar PWM, in PWM/PPM, the envelope magnitude is encoded in the pulse width and the phase information is mapped to the position of the pulse, which is encoded by PPM. The difference is that, to fit the limited sampling rate of digital system, e.g., a few times the RF carrier frequency, and to meet the requirement of linearity, both envelope magnitude and phase signals should be noise-shaped first by band-pass delta sigma function, and then processed by the PWM/PPM. But the noise-shaping degrades the power coding efficiency dramatically.
Hence, there is a demand for a high-efficiency new power coding scheme, particularly the capability of digital implementation for the direct digital-RF transmitter architecture.